1. Field of the Invention
The present invention relates to filter assemblies and optical modules for use, e.g., in optical transceivers for transmitting gigabit Ethernet (registered trademark) signals.
2. Description of Related Art
In recent years, the Internet has become established as a communication infrastructure regardless of the type of information such as data, audio and video; hence it has attracted various businesses and is still continuing to expand its versatility. With this advance, the available bandwidths (i.e., communication capacity) are also continually on the rise. Among high-speed computer networking technologies, the Ethernet (registered trademark) is low cost and easy to manage, and is thereby in wide spread use as a core technology for a home LAN, a WAN, etc.
In this trend, the 10 Gigabit Ethernet (registered trademark) has already been standardized, and various companies have developed networking devices for use in it. Following this, optical transceivers have begun to be upgraded from 1 Gigabit Ethernet compatible to 10 Gigabit Ethernet compatible mainly in the field of intermediate-area networks.
Such optical transceivers include LX4 optical transceivers for four-channel wavelength division multiplexing (WDM) systems using four long-wavelength semiconductor laser diodes (LDs). The LX4 optical transceivers comprise a base, plural optical devices and plural optical filters in which: the base includes the plural optical devices and filters; the plural optical devices emit or receive optical signals of different wavelength; and the plural optical filters multiplex plural optical signals of different wavelength into a wavelength multiplexed optical signal or divide a wavelength multiplexed optical signal into plural optical signals of different wavelength.
Particularly, an optical module which mounts, on a base, semiconductor laser diodes (LDs) serving as optical devices and plural optical filters serving as an optical multiplexer is called a transmitting optical sub-assembly (TOSA). While, an optical module which mounts, on a base, photodiodes (PDs) serving as optical devices and plural optical filters serving as an optical demultiplexer is called a receiving optical sub-assembly (ROSA). Conventionally, plural edge filters are used as the optical filters, and are individually and independently mounted on a filter mount of a base while being positionally adjusted (aligned along the optical axis).
For example, JP-A-2005-140960 discloses an optical device having a cylindrical casing in which five edge filters serving as wavelength selection means are substantially equally spaced on the casing, along respective optical paths within the casing, and in front of respective ones of five light receiving means provided outside the casing, as most clearly shown in the first figure of JP-A-2005-140960. Also, JP-A-2002-72010 discloses an optical device, which has a multi-layer film filter with a continuously varying thickness across its surface, and a wavelength multiplexed optical signal is obliquely incident on the filter of the optical device to separate the optical signal.
As described above, in conventional methods of mounting optical filters to an optical transmission assembly, plural edge filters are individually and independently mounted thereto so as to provide a wavelength multiplexing function. Such conventional methods have many problems. For example, such optical filters require relatively many operations for assembling, and they are prone to misalign with the optical axis after alignment when the ambient temperature changes, thereby causing variation in the optical output power level.
In conventional methods of mounting optical filters to an optical receiving assembly, a single integrated Can package capable of accommodating all the plural PDs may be used, but a PD used in a general-purpose Can-package can not be applied in such a custom-made integrated package, resulting in an increase in the cost of optical receiving assemblies and optical transceivers. Although a PD array may be used as the plural PDs, in this case it is difficult to form a compact optical filter unit, thus incurring an increase in the size of optical receiving assemblies and therefore optical transceivers. There is another problem in that conventional optical transceivers have a relatively large number of parts to be managed because a separate individual optical filter is needed for each wavelength.
In the case of the filter assembly formed of a laminated multi-layer optical film filter described in the above-mentioned JP-A-2002-72010, general-purpose Can-packaged PD can not be used because of the small pitch between adjacent exits of wavelength separated optical signals, thus causing an increase in the entire cost of the assembly. Further, the incident angle of a wavelength multiplexed optical signal on the filter assembly is limited, thus reducing the design flexibility of the base and optical assembly.